It is well known to provide electrical systems which incorporate both high power electronic and low power electronic components in a single design. For efficiency of space, it is often desirable to incorporate both kinds of components into a single housing unit but this can create problems due to electromagnetic interference. Hence, it is common practice to split the high and low power devices into separate housings.
For example, in electric power assisted steering systems of the kind set forth, the electric motor is used to assist a driver of the vehicle in turning the steering column, by applying an assistance torque of the same sense as the torque applied by the driver, which makes it easier to turn the steering wheel, for example during parking. The motor is controlled by motor control circuitry and drive circuitry. The control circuitry is typically connected to a torque sensor associated with the steering column and adapted to provide a signal indicative of the torque demand by the driver. The motor drive circuitry is adapted to switch the motor drive currents between, for example, different phases in the motor under the control of the motor control circuitry. It typically comprises a plurality of high power transistors connected in a bridge as it must handle the relatively high currents demanded by the motor. The transistors are switched under the control of the control circuitry.
Electric motors are known to be relatively strong sources of electromagnetic radiation which can cause interference problems with other electronic circuitry provided in the motor vehicle and, in particular with the control circuitry in the steering system. Furthermore, because of the limited choice of positions in which the steering system can be mounted within the increasingly cramped confines of a vehicle, it is often exposed to many other sources of electromagnetic radiation. External sources of radiation, such as overhead power lines, can also produce electromagnetic radiation which can cause interference. In a safety critical application such as a power-assisted steering system, it is important to provide a high level of shielding from this electromagnetic radiation to minimise the susceptibility of the electronic control and drive circuitry from interference.
It is already known to overcome the problem of electromagnetic interference by providing the drive and control circuitry inside separate conductive casings which are bolted onto a main housing body which contains the gearbox. Another casing is provided which houses the motor. The purpose of providing the separate casings is to isolate the electronic circuitry from the motor and gearbox. However, each casing requires a separate cover or other opening to allow installation and maintenance. These openings provide potential points of leakage and entrance of radiation through the various associated seals.